<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>函数写阶乘</title>
</head>
<body>
  <script>
    var num = 1
    function fn(n) {
      for(let i = 1; i <= n; i++) {
        num *= i // num *= i 等于 num = num * i
      }
      return num
    }
    console.log(fn(5))
  </script>
</body>
</html>